Subterranean deposits comprising both sodium carbonate and sodium bicarbonate vary in composition from one location to another as might be expected, however, the major commercially developable deposits generally have one of two basic compositions. One of the naturally-occurring sodium (bi)carbonate mineral is known as “Wegscheiderite” and may be also called “decemite”. This mineral contains 29.6% Na2CO3 and 70.4% NaHCO3 by weight in the form of three molecules of NaHCO3 for each molecule of Na2CO3 as follows: Na2CO3.3NaHCO3.
The second sodium (bi)carbonate mineral is the naturally-occurring mineral called “trona”. Crude trona is a mineral that may contain up to 99% of sodium sesquicarbonate (generally about 70-99%). Sodium sesquicarbonate is a sodium carbonate—sodium bicarbonate double salt having the formula (Na2CO3.NaHCO3.2H2O) and which contains 46.90 wt. % Na2CO3, 37.17 wt. % NaHCO3 and 15.93 wt. % H2O. Crude trona also contains, in lesser amounts, sodium chloride (NaCl), sodium sulfate (Na2SO4), organic matter, and insolubles such as clay and shales. A typical analysis of the trona ore mined in Green River is shown in TABLE 1. Trona is important from a commercial standpoint in the United States at least due to the very large deposits in the State of Wyoming. The most valuable alkali produced from trona is sodium carbonate (Na2CO3). Sodium carbonate is one of the largest volume alkaline commodities produced in the United States. In 2007, trona-based sodium carbonate from Wyoming comprised about 91% of total U.S. soda ash production. Sodium carbonate finds major use in the glass-making industry and for the production of baking soda, detergents and paper products. Other products such as sodium bicarbonate (NaHCO3), sodium sulfite (Na2SO3), caustic soda (NaOH), sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O), a sodium phosphate (Na5P3O10) or other sodium-containing chemicals may be produced from trona as well.
TABLE 1ConstituentWeight PercentNa2CO343.2-45NaHCO333.7-36H2O (crystalline and free moisture)  15.3-15.6NaCl0.004-0.1 Na2SO4 0.005-0.01Insolubles  3.6-7.3
Most mining operations practiced some form of underground ore mechanical extraction using techniques adapted from the coal mining industry. A variety of different systems and mining techniques (such as longwall mining, shortwall mining, room-and-pillar mining, or various combinations) exist for mechanically mining ore containing sodium (bi)carbonate from underground formations. The large deposits of mineral trona in southwestern Wyoming near Green River Basin have been mechanically mined since the late 1940's and have been exploited by five separate mining operations over the intervening period. The nominal depth below surface of these mining operations ranges between approximately 800 feet to 2000 feet. Although any of these various mining techniques may be employed to mine trona ore, when a mechanical mining technique is used, it is preferably longwall mining.
To recover these valuable alkali products, the so-called ‘monohydrate’ commercial process is frequently used to produce soda ash from trona. When the trona is mechanically mined, crushed trona ore is calcined (i.e., heated) to convert sodium bicarbonate into sodium carbonate, drive off water of crystallization and form crude soda ash. The crude soda ash is then dissolved in water and the insoluble material is separated from the resulting solution. A clear solution of sodium carbonate is fed to a monohydrate crystallizer, e.g., a high temperature evaporator system generally having one or more effects (sometimes called ‘evaporator-crystallizer’), where some of the water is evaporated and some of the sodium carbonate forms into sodium carbonate monohydrate crystals (Na2CO3.H2O). The sodium carbonate monohydrate crystals are removed from the mother liquor and then dried to convert the crystals to dense soda ash. Most of the mother liquor is recycled back to the evaporator system for additional processing into sodium carbonate monohydrate crystals.
Longwall mining, shortwall mining, and room-and-pillar mining require miners and heavy machinery to be underground. The cost of the mechanical mining methods for trona is high, representing as much as 40 percent of the production costs for soda ash. Furthermore, recovering trona by these methods becomes more difficult as the thickest beds (more readily accessible reserves) of trona deposits with a high quality (less contaminants) were exploited first and are now being depleted. Thus the production of sodium carbonate using the combination of mechanical mining techniques followed by the monohydrate process is becoming more expensive, as the higher quality trona deposits become depleted and labor and energy costs increase. Furthermore, development of new reserves is expensive, requiring a capital investment of as much as hundreds of million dollars to sink new mining shafts and to install related mining and safety (ventilation) equipment.
Recognizing the economic and physical limitations of underground mechanical mining techniques, various solution mining techniques have been proposed. Solution mining allows the recovery of sodium values from underground formation comprising water-soluble ore without the need for sinking costly mining shafts and employing workers in underground mines. In its simplest form, solution mining comprises injecting water or an aqueous solution into a cavity of the underground formation, allowing the solution to dissolve as much water-soluble ore as possible, pumping the resulting brine to the surface, and recovering the dissolved ore from the brine.
With respect to ores containing sodium carbonate and sodium bicarbonate (sometimes termed ‘sodium (bi)carbonate-containing ore), while trona is more soluble than Wegscheiderite in water at room temperature, these ores are still of relatively low solubility when compared with other naturally-occurring minerals mined “in situ” with solution mining techniques, such as halite (mostly sodium chloride) and potash (mostly potassium chloride). Thus implementing a solution mining technique to exploit sodium (bi)carbonate-containing ores, especially those that would not be economically viable to mine via mechanical mining techniques, is quite challenging.
A first effort can be found in a solution mining technique proposed in U.S. Pat. No. 2,388,009 to Pike. Pike discloses a method of producing soda ash from underground trona deposits in Wyoming by injecting a heated brine containing substantially more carbonate than bicarbonate which is unsaturated with respect to the trona, withdrawing the solution from the formation, removing organic matter from the solution with an adsorbent, separating the solution from the adsorbent, crystallizing, and recovering sodium sesquicarbonate from the solution, calcining the sesquicarbonate to produce soda ash, and re-injecting the mother liquor from the crystallizing step into the formation.
Another patent to Pike, U.S. Pat. No. 2,625,384, discloses another solution mining method which uses water as a solvent under ambient temperatures to extract trona from existing mined sections of the trona deposits. The subsequent solution is withdrawn from the mine and heated before dissolving additional dry mined trona in the solution to form a carbonate liquor having more concentrated values of sodium salts which can subsequently be processed into sodium carbonate.
However, the solution mining process for a sodium (bi)carbonate-containing ore is not as simple as is the case with solution-mining of single-salt ores such as to recover sodium chloride or potassium chloride because of the complex solubility relationships in the ore containing sodium sesquicarbonate (main component of trona) or wegscheiderite. A complicating factor in dissolving in situ these types of underground double-salt ores is that sodium carbonate and sodium bicarbonate have different solubilities and dissolving rates in water. These incongruent solubilities of sodium carbonate and sodium bicarbonate can cause sodium bicarbonate “blinding” (sometimes termed ‘bicarb blinding’) during solution mining.
Blinding occurs as the bicarbonate, which has dissolved in the mining solution tends to redeposit out of the solution onto the exposed face of the ore as the carbonate saturation in the solution increases, thus clogging the dissolving face and “blinding” its carbonate values from further dissolution and recovery. Blinding can thus slow dissolution and may result in leaving behind significant amounts of reserves in the mine.
It can be shown that the aforementioned problem arises because when trona, for example, is dissolved in water, both the sodium bicarbonate and the sodium carbonate fractions begin going into solution at the same time until the solution reaches saturation with respect to sodium bicarbonate. Unfortunately, the resulting liquid phase existing at this point is in equilibrium with sodium bicarbonate in solid phase, and the sodium carbonate continues to dissolve while the bicarbonate starts precipitating out until the final resulting solution is in equilibrium condition with sodium sesquicarbonate (trona) as the stable solid phase, in fact, reached wherein a substantial portion of sodium bicarbonate precipitates out of solution and a lot more of the sodium carbonate has gone into solution. Wegscheiderite behaves in much the same way as trona in that they both go into solution in accordance with their respective solid percentage compositions of sodium bicarbonate and sodium carbonate, however, more sodium carbonate wants to go into solution and, because of this, it causes part of the sodium bicarbonate to precipitate back out. The resulting equilibrium condition is one in which substantially more sodium carbonate and a good deal less sodium bicarbonate exists in the solution phase than was present in the original solid phase mineral composition.
It is this phenomenon of the unstable nature of both trona and Wegscheiderite in solution in the presence of the solid phase mineral that is responsible for the clogging problem. More specifically, the sodium bicarbonate that precipitates out does so upon the surrounding, thus producing a barrier that inhibits the solvent action of the water upon the more water-soluble sodium carbonate trapped and sealed underneath the re-deposited sodium bicarbonate. The net result of this phenomenon is to progressively change the effective composition of the formation upon which the aqueous solvent acts until it appears to be made up of sodium bicarbonate alone. In other words, as more and more of the sodium bicarbonate precipitates out, this deposit seals off the interstices through which the aqueous solvent can gain access to the sodium carbonate in the formation, thereby permitting the aqueous solvent to act upon successively smaller amounts of sodium carbonate until about all the aqueous solvent can reach is the sodium bicarbonate barrier itself. As previously stated, both of the naturally-occurring sodium (bi)carbonate-containing minerals (namely, wegscheiderite and trona) behave in the same way. Nahcolite, a mineral which contains mainly sodium bicarbonate, does not suffer from such phenomenon due to the fact that nahcolite is essentially free of sodium carbonate.
Therefore it is expected that long term solution mining of a sodium (bi)carbonate-containing mineral may produce brines with lower sodium carbonate values and higher sodium bicarbonate values than those seen initially. This requires that a process be capable of handling the changing brine grade or that incongruent dissolution must be avoided by some means.
“Bicarb blinding” is an occurrence which has been recognized as a problem pertaining to solution mining of trona. Methods to address such phenomenon are described, for example, in some U.S. patents. U.S. Pat. No. 3,184,287 to Gancy discloses a method for preventing incongruent dissolution and bicarbonate blinding in the mine by using an aqueous solution of an alkali, such as sodium hydroxide having a pH greater than sodium carbonate, as a solvent for solution mining. In US '287, the aqueous sodium hydroxide solvent used in trona solution mining is regenerated by causticization of aqueous sodium carbonate with lime.
U.S. Pat. No. 3,953,073 to Kube and U.S. Pat. No. 4,401,635 to Frint also disclose solution mining methods using a solvent containing sodium hydroxide. US '073 describes the use of aqueous sodium hydroxide for solution mining of trona and nahcolite, and of other NaHCO3-containing ores, and discloses that the solvent requirements may be met either by causticization of soda ash with hydrated lime or by the electrolytic conversion of sodium chloride to sodium hydroxide.
U.S. Pat. No. 4,652,054 to Copenhafer et al. discloses a solution mining process of a subterranean trona ore deposit with electrodialytically-prepared aqueous sodium hydroxide in a three zone cell in which soda ash is recovered from the withdrawn mining solution.
U.S. Pat. No. 4,498,706 to Ilardi et al. discloses the use of electrodialysis unit co-products, hydrogen chloride and sodium hydroxide, as separate aqueous solvents in an integrated solution mining process for recovering soda ash. The electrodialytically-produced aqueous sodium hydroxide is utilized as the primary solution mining solvent and the co-produced aqueous hydrogen chloride is used to solution-mine NaCl-contaminated ore deposits to recover a brine feed for the electrodialysis unit operation.
These patents are hereby incorporated by reference for their teachings concerning solution mining with an aqueous solution of an alkali, such as sodium hydroxide and concerning the making of a sodium hydroxide-containing aqueous solvent via electrodialysis.
Unfortunately, to avoid incongruent dissolution, alkalis such as sodium hydroxide or lime need to be used constantly during solution mining, and because of their high costs, such constant use adversely affects the economics of such solution mining processes.
The present invention thus provides a remedy to some of the problems associated with ‘bicarb blinding’ during solution mining of trona and/or wegscheiderite.
Although this foregoing ‘bicarb blinding’ issue has been and will be described in terms of trona and/or wegscheiderite mining, it may also apply to solution mining of any double-salt ore with incongruent solubilities.